1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an electrode structure of an active matrix type crystal display device drived by a horizontal electric field.
2. Description of the Prior Art
A twisted nematic (TN) system, which has heretofore been used widely, has a problem that it has a remarkable viewing angle dependency while it has a high contrast because a molecular axis of liquid crystal rises by vertical electric field. In recent years, demand for the liquid crystal display device for use in large-scale monitor such as TVs has been increased, and an In-Plane Switching (IPS) system has been increasingly adopted. In the IPS system, displaying is performed by rotating a molecular axis of liquid crystal on a plane in parallel with a substrate by means of horizontal electric field. Since the IPS system eliminates the viewing angle dependency on a rise-up angle of the molecular axis, the IPS system is more advantageous in its viewing angle property than TN system.
However, in the IPS system, pixel electrodes and common electrodes are formed in a comb shape to apply horizontal electric field. Therefore, the IPS system has problems that a ratio of an electrode area to a display area is high and a ratio of aperture is low.
An example in which such a low aperture ratio of the IPS system is improved is disclosed in Japanese Patent Laid-Open No. 2002-323706. FIG. 1 is a plan view of a liquid crystal display device of an IPS system disclosed in Japanese Patent Laid-Open No. 2002-323706. FIGS. 2A to 2C are section views of a substrate comprising thin film transistors (TFT) (hereinafter referred to as a TFT substrate), which are taken along the lines I-I, II-II and III-III of FIG. 1, respectively.
Referring to FIG. 1 and FIGS. 2A to 2C, a scanning signal wiring 401 made of a first metallic layer and a common signal wiring 402 in parallel therewith are formed on a transparent insulating substrate 400 such as a glass plate. An insulating film 403 is formed on the scanning signal wiring 401 and the common signal wiring 402. A video signal wiring 404 made of a second metallic layer, a TFT 405 (indicated by dotted lines) and a pixel electrode 406 are formed on the insulating film 403. An insulating film 407 is formed on the video signal wiring 404, the TFT 405 and the pixel electrode 406.
Furthermore, an insulating film 408 is coated on the entire of the insulating film 407. On this insulating film 408, a pixel electrode 409 formed of a transparent electrode and a common electrode 410 are formed. The pixel electrode 409 and the common electrode 410 are formed in a comb shape. Note that the arrow solid line of FIG. 1 indicates a rubbing direction of an alignment film (not shown) for defining an initial alignment of liquid crystal.
The video signal wiring 404 is fully covered with the common electrode 410 while interposing the insulating films 407 and 408 therebetween. The pixel electrode 409 and the common electrode 410 are electrically connected to a source electrode 406A and the common signal wiring 402 through contact holes 411 and 412, respectively.
As described above, since the pixel electrode 409 and the common electrode 410, which are comb-shaped, are formed of a transparent electrode, an area on the electrodes also contributes to a ratio of aperture. According to a simulation, if a contribution amount on the transparent electrode is considered, an effective ratio of aperture increases by about 8%. Because of the structure in which the video signal wiring 404 is covered with the common electrode 410, it is possible to widen an aperture portion to the vicinity of the video signal wiring 404. Note that such a constitution generates a load-carrying capacity between the video signal wiring 404 and the common electrode 410. However, since the common electrode 410 is formed on the video signal wiring 404 with the low dielectric insulating film interposed therebetween, the load-carrying capacity can be controlled within the range of no driving problems.
An example in which a high aperture ratio in an IPS liquid crystal display device is further increased is proposed in Japanese Patent Laid-Open No. 2004-062145. FIG. 3 is a plan view of the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145. FIGS. 4A to 4C are section views of a TFT substrate taken along the line I-I, II-II, and III-III of FIG. 3, respectively. Referring to FIG. 3 and FIGS. 4A to 4C, a scanning signal wiring 501 formed of a first metallic layer and a common signal wiring 502 in parallel therewith are formed on a transparent insulating substrate 500 such as a glass plate. An insulating film 503 is formed on the scanning signal wiring 501 and the common signal wiring 502. On the insulating film 503, a video signal wiring. 504 formed of a second metallic layer, a TFT 505 (indicated by a circle of the dotted line) and a pixel electrode 506 integrated with a source electrode are formed. Note that the arrow solid line of FIG. 3 indicates a rubbing direction of an alignment film (not shown) for defining an initial alignment of liquid crystal.
Since the source electrode is formed integrally with the pixel electrode 506, a contact hole for connecting the pixel electrode 506 to the source electrode is unnecessary. An insulating film 507 is formed on the video signal wiring 504, the TFT 505 and the pixel electrode 506 integrated with the source electrode. Furthermore, an insulating film 508 is entirely coated onto the insulating film 507. When a common electrode 510 formed of a transparent electrode is formed on the insulating film 508, a driving voltage is significantly increased because the pixel electrode 506 and the common electrode 510 are formed in a lattice shape such that a plurality of vertical bars are extended between a pair of a horizontal bars, respectively, as shown in FIG. 3 and FIG. 4. Accordingly, the insulating film 508 is left only above the video signal wiring 504 so as to be bank-shaped for the purpose of reducing the load-carrying capacity. Thereafter, the common electrode 510 formed of the transparent electrode is formed. The common electrode 510 is electrically connected to the common signal wiring 502 through the contact hole 512. It does not matter whether the insulating film 508 may be transparent or colored.
Same as in the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2002-323706, in the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145, the video signal wiring 504 is covered with the common electrode 510 while interposing the insulating film 508 therebetween. However, in the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145, the common electrode is formed to be bank-shaped, and the video signal wiring 504 has a shape of a cross section surrounded by the common electrode. Accordingly, a shield effect for electric field leaking from the video signal wiring 504 of the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145 is stronger than that of the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2002-323706.
As described above, in the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145, the number of the contact holes decreases by one, and a width of a flare of the common electrode extending from an edge of the video signal wiring can be made smaller. Therefore, a higher aperture ratio can be obtained. In the IPS liquid crystal display device, since the pixel electrode is formed of the metallic electrode, a contribution amount to the aperture ratio on the pixel electrode is less than the prior art. However, the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145 increases its effective aperture ratio by about 5%, and has an aperture ratio higher than that of the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2002-323706. Also a vertical crosstalk is suppressed.
In the IPS liquid crystal display devices disclosed in Japanese Patent Laid-Open Nos. 2002-323706 and 2004-062145, when a pixel pitch is large, it is possible to obtain an effective aperture ratio close to that of a TN liquid crystal display device. However, when the pixels are arrayed in a narrower pitch, a size of the contact hole between the common electrode which is the upper layer and the common signal wiring which is the lower layer creates a problem even in the case of the IPS liquid crystal display device disclosed in Japanese Patent Laid-Open No. 2004-062145. Therefore, since the pixel electrode has to be disposed so as to make a detour with a margin from the contact hole, this is contributory to damage the aperture ratio. Since there is a limit to a compact size of the contact hole, the narrower the pitch of the pixel becomes, the less ignorable the influence of the contact hole on the aperture ratio is.